A. Field of the Invention
The present invention relates to tie rods of the type used to reinforce ventilating ducts which convey air for heating, ventilating or air conditioning (HVAC) the interior spaces, of buildings or similar structures. Such tie rods are used to prevent excessive flexure of duct walls in response to fluctuations of air pressure exerted on walls of the duct More particularly, the invention relates to a method and apparatus for manufacturing self-sealing reinforcement tie rods for ventilating ducts.
B. Description of Background Art
Ductwork used to heat, ventilate, or air condition buildings usually consists of elongated lengths of tubing comprising individual duct sections which are coupled together to form a continuous, air-tight duct for conveying flowing air. Typical duct tubing is made of a material such as sheet metal which is relatively strong but sufficiently light in weight to minimize the size and weight of fasteners and structural components required to support the ductwork, as well as maintaining materials and fabrication costs of the ductwork itself at reasonable levels. Thus, typical ducts are made of relatively thin sheet steel ranging in thickness from 26 gauge (0.0188 inch thick) to 16 gauge (0.0625 inch thick).
Sheet metal ducts have rectangular, circular or oval cross section shapes, and are often manufactured and supplied in precut lengths or sections with transversely outwardly protruding interconnection flanges provided at opposite longitudinal ends of the section, to facilitate interconnecting duct sections at a job site and thus forming air conveying ducts of desired lengths and orientations.
Ducts of the tape described above are available in a wide variety of sizes, having cross-section dimensions which range from a few inches to several feet. Moreover, a wide range of width-to-height or aspect ratios of rectangular ducts are available. For example, a typical 18 inch high duct may have a width in the range of 2 feet to 4 feet, and a length of 5 feet. Whatever the shape, size and aspect ratio of the duct, the relatively small thickness of its walls relative to its cross-sectional dimensions results in the duct walls being relatively flexible. Thus, conventional ducts may experience relatively large, possibly destructive deformations if static or dynamic differential air pressure between the interior and exterior of the duct exceeds predetermined threshold values. For this reason, mechanical engineering standards as well as applicable building codes require that duct work used to conduct air in certain heating, ventilating, and air conditioning (HVAC) applications be reinforced against expansion when positively pressurized and/or against collapse when negatively pressurized.
A widely employed reinforcement method that meets code requirements consist of installing elongated straight, rigid reinforcement members within a duct at pre-determined spacings which depend upon the rigidity of the duct, and upon the maximum differential pressures which it may encounter. Such reinforcement members are disposed perpendicularly between inner surfaces of opposite walls of the duct and fastened at opposite ends thereof to those walls. One such approved reinforcement member which is in current use consists of a threaded steel tie rod which has installed onto each end thereof a first, inner nut which is threadingly advanced to a predetermined distance inward from the end of the rod. The distance between the outer faces of the inner nuts is made equal to the minimum cross-sectional dimension, i.e., the height of the duct. The tie rod is positioned within a duct perpendicularly between a pair of opposed walls of the duct, and opposite ends of the threaded rod are each inserted through a separate one of a pair of transversely or vertically aligned holes provided through the duct walls. A pair of external nuts are then threaded onto the opposite ends of the threaded rod which protrude outwardly through the duct wall holes. Each external nut is then tightened onto the threaded rod against the outer surface of the duct wall, while the adjacent inner nut is held against rotation with a separate wrench. For large ducts, this operation requires two workmen.
Another prior-art duct reinforcement uses an elongated tube containing in opposite ends of the bore openings thereof an internally threaded fastener member which is fixed in the tube and which is adapted to receive a machine screw inserted through a duct wall hole. This reinforcement method also sometimes requires that the tube being gripped while the external machine screw is torqued, which again may require two workmen.
A third type of prior art tie rod used to reinforce HVAC ducts, which is a variation of the first method, uses a tube which encloses a threaded rod, the latter being secured between the walls of the duct by two external nuts, and is no easier to install than the other two types described above.
In addition to being somewhat difficult and time consuming to install, in accordance with certain code requirements, prior art reinforcement tie rods of the type discussed above must make an air-tight seal with the duct wall holes through which they protrude, thus requiring installation of a resilient sealing element such as a gasket or washer between the exterior nut or screw head and the duct wall. If the exterior nut or screw is torqued too tightly, such resilient elements can be deformed or damaged, and may ultimately fail to achieve sealing.
Motivated by limitations of prior art rod fasteners of the type discussed above, the present inventor developed a self-sealing reinforcement tie rod for reinforcing ventilating ducts. That tie rod was disclosed in the present inventors U.S. patent application Ser. No. 09/307,270, filed May 7, 1999, now U.S. Pat. No. 6,116,833. The novel reinforcement tie rod disclosed in the foregoing reference includes a length of threaded steel rod which is slightly longer, e.g., about 1 1/4 inches, longer than the height of ducts which the tie rod is intended to reinforce. Thus, for use in 12-inch high ducts, the threaded rod portion of the tie rod according to the previous invention may have a typical diameter of about 3/8-inch and a length of about 13 1/4 inches. The tie rod according to the prior invention also includes a pair of abutment structures fastened to opposite end portions of the threaded rod, inwards of the outer transverse end walls of the rod. Each abutment structure includes a longitudinally inwardly located tubular part which receives the threaded rod, and is permanently fastened thereto by, for example, crimping the wall of the tubular section into the threads of the rod. Each abutment structure also includes at longitudinally outwardly located annular flange which is disposed transversely to the axis of the threaded rod. The longitudinal spacing between the outer transverse walls of the abutment structure flanges is made nearly equal to the height of the duct. Thus, when the rod is placed inside a duct and opposite ends of the rod are inserted outwardly through a pair of vertically aligned holes made in the upper and lower walls of the duct, the outer transverse surfaces of the two abutment structure flanges abut the opposed inner duct wall surfaces, and each end of the rod protrudes about 5/8-inch outwardly from the adjacent duct wall.
According to the prior invention, the outer longitudinal portion of the bore in the tubular part of each abutment structure flares arcuately outwardly to join the outer transverse flange wall of the abutment structure, forming an annular cavity between the flange wall surface and threaded rod. The cavity has a diameter which tapers smoothly radially and longitudinally outwardly towards the flange. Prior to installation of the tie rod in a duct, a pair of resilient O-rings having an inner diameter slightly less than that of the threaded rod are slipped onto opposite outer ends of the rod.
The tie rod according to the prior invention also includes a pair of internally threaded fasteners located externally to the duct, which are threaded onto opposite ends of the threaded rod which protrude outwardly through the duct walls. In the preferred embodiment, each external threaded fastener consists of a flare nut having an outer hexagonal head and an inner annular skirt flange which flares outwardly from the head, the skirt having a flat lower or inner surface. When the flare nut is threaded down onto the protruding length of the threaded rod, and tightened down onto the outer surface of a duct wall, the O-ring is compressed between the inner surface of the duct wall and the outer transverse wall of an abutment structure flange. Further tightening of the flange nut on the threaded rod causes the O-ring to cold flow partially into the arcuately curved annular cavity between the flange wall and rod, and against the outer cylindrical wall surface of the threaded rod, thus forming a highly effective hermetic seal of the hole through the duct wall.
The present invention provides an efficient method and apparatus for manufacturing self-sealing reinforcement tie rods of the type described above.